Fix OPL MIDI tempo calculations.

It turns out that the way that tempo has been calculated in OPL playback
has been broken for a long time. The mysterious "fudge factor" that I
had to apply to tempo calculations is actually completely unnecessary:
the byte-swapping in the MIDI_GetFileTimeDivision() function was being
done wrong, so the time division being used by the OPL MIDI code was
completely wrong. Presumably the multiply by 260 was close enough to an
8-bit bitshift that it worked okayish, but large enough time division
values would overflow a single byte and screw up.

This fixes long-running OPL playback problems in a number of WADs, most
notably Alien Vendetta. This *really* fixes #352.